PURPOSE: To embody a duplex device which can be shared as an individual power feeding type and a common power feeding type, in a duplex device connection system in a duplex system.

CONSTITUTION: Two pairs of duplex devices 300 which are operated as a persent system and a stand-by system are connected through one of first and second confounding lines 100 and 200, and a swetching control circuit 301 in each duplex device mutually reports a signal to indicate to which system of the present system or the stand-by system its own device belongs through the connected confounding line. This system is equipped with a selecting means 302 which selects the designated confounding line when one of the first and second confounding lines 100 and 200 to be connected to the circuit 301 is designated, and transmits the signal transmitted from the other duplex device through the selected confounding line, and a distributing means 303 which transmits the signal transmitted from the switching control circuit 301 to the both connected confounding lines 100 and 200.